The process of blood vessel formation is fundamental in both development and disease. The circulatory system is the first organ system to emerge during embryogenesis, and is necessary to nourish the developing fetus. Disorders of the circulatory system, such as coronary artery disease, are a major cause of morbidity and mortality in modern society. Thus, repairing, replacing and promoting the growth of blood vessels is a major target of clinical research and of pharmaceutical development. Conversely, the ingrowth of new capillary networks into developing tumors is essential for the progression of cancer. Thus, the development of drugs that inhibit this process of tumor angiogenesis is an equally important therapeutic goal. Little attention has been paid to the problem of how arteries and veins acquire their distinct identities. Indeed, many people have assumed that the anatomical and functional differences between arteries and veins simply reflect physiological influences, such as blood pressure, oxygenation and shear forces. Additional knowledge of how arteries and veins acquire their respective identities would be valuable in both research and clinical settings.